1. Field of the Invention
This invention relates generally to a perpendicular spin-transfer-torque magnetic-random-access memory (MRAM) cell having permeable dielectric layers for reducing external perpendicular stray field interference.
2. Description of the Related Art
In recent years, magnetic random access memories (hereinafter referred to as MRAMs) using the magnetoresistive effect of ferromagnetic tunnel junctions (also called MTJs) have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can also cope with high-speed reading and writing. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulating tunnel barrier layer, and a fixed layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction. Corresponding to the parallel and anti-parallel magnetic states between the recording layer magnetization and the reference layer magnetization, the magnetic memory element has low and high electrical resistance states, respectively. Accordingly, a detection of the resistance allows a magnetoresistive element to provide information stored in the magnetic memory device.
There has been a known technique for achieving a high MR ratio by forming a crystallization acceleration film that accelerates crystallization and is in contact with an interfacial magnetic film having an amorphous structure. As the crystallization acceleration film is formed, crystallization is accelerated from the tunnel barrier layer side, and the interfaces with the tunnel barrier layer and the interfacial magnetic film are matched to each other. By using this technique, a high MR ratio can be achieved.
Typically, MRAM devices are classified by different write methods. A traditional MRAM is a magnetic field-switched MRAM utilizing electric line currents to generate magnetic fields and switch the magnetization direction of the recording layer in a magnetoresistive element at their cross-point location during the programming write. A spin-transfer torque (or STT)-MRAM has a different write method utilizing electrons' spin momentum transfer. Specifically, the angular momentum of the spin-polarized electrons is transmitted to the electrons in the magnetic material serving as the magnetic recording layer. According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to the magnetoresistive element. As the volume of the magnetic layer forming the recording layer is smaller, the injected spin-polarized current to write or switch can be also smaller. In a so-called perpendicular STT-MRAM, both two magnetization films in an MTJ stack have easy axis of magnetization in a direction perpendicular to the film plane due to their strong magnetic crystalline anisotropy and interface interaction induced anisotropy, shape anisotropies are not used, and accordingly, the device shape can be made smaller than that of an in-plane magnetization type. Also, variance in the easy axis of magnetization can be made smaller. Accordingly, both miniaturization and lower currents can be expected to be achieved while a thermal disturbance resistance is maintained. In another word, perpendicular STT-MRAM having high speed, large capacities and low-power-consumption operations can potentially replace the conventional semiconductor memory used in electronic chips, especially mobile chips for power saving and non-volatility.
Reading STT MRAM involves applying a voltage to the MTJ stack to discover whether the MTJ element states at high resistance or low. However, a relatively high voltage needs to be applied to the MTJ to correctly determine whether its resistance is high or low, and the current passed at this voltage leaves little difference between the read-voltage and the write-voltage. Any fluctuation in the electrical characteristics of individual MTJs at advanced technology nodes could cause what was intended as a read-current, to have the effect of a write-current, thus reversing the direction of magnetization of the recording layer in MTJ.
The thermal stability of the magnetic orientation in a MRAM cell is a critical parameter which has to be kept high enough for a good data retention, and is typically characterized by the so-called thermal factor which is proportional to the perpendicular anisotropy as well as the volume of the recording layer cell size.
Combining writing, reading and thermal stability factors, an MRAM has to be well designed and manufactured with tight processing variations. However, in a real application, especially in mobile application, there is risk that an MRAM chip gets close to an external field, which could have potentially interference with data recording, yielding incorrect data writing, or interference with reading, yielding unwanted data writing. In an in-plane STT MRAM, this can be easily addressed by adding a soft magnetic shield above MRAM array. It becomes very challenging in a perpendicular STT MRAM design, since a planar shield only provides shielding effect on in-plane field and there is no shielding effect on an external perpendicular stray field.
Thus, it is desirable to provide perpendicular STT-MRAM structures having a local shielding to reduce a perpendicular stray field to very low level to achieve a good data retention without data destruction by external stray field.